The present invention relates to a surface acoustic wave (SAW) filter using a novel piezoelectric single crystal substrate, and more particularly to a high frequency SAW filter with a low propagation loss prepared by using a single crystal wafer of a composite metal material having a high dielectric constant, high piezoelectric constant, and good electromechanical properties.
A surface acoustic wave (SAW) filter is a passive device designed to selectively pass desired signals and to suppress undesired signals in the electronic and telecommunication system. A SAW filter typically comprises a piezoelectric substrate in the form of a single crystal and input and output interdigital transducer (IDT) electrodes formed on the substrate, as shown in FIG. 1. When an electrical signal is applied to one of the two IDT electrodes, the piezoelectric substrate is stressed to generate a surface acoustic wave which propagates on the surface of the piezoelectric substrate and which is extracted as an electrical signal at the other IDT electrode.
In a SAW filter, the passband frequency is determined by both the velocity, of the surface acoustic wave generated on the substrate and the spacing between the two fingers of IDT, and it is typically represented by a center frequency, f0, which is a ratio V/xcex0, V and xcex0 representing the propagation velocity of the surface acoustic wave and the finger spacing, respectively.
In conventional SAW filters which employ quartz, lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) single crystals as a piezoelectric substrate and aluminum as an IDT material, the propagation velocities of the surface acoustic waves are reported to be 3,157 m/sec, 4,178 m/sec and 4,742 m/sec, respectively.
There have been hitherto attempted to increase the passband frequency of a SAW filter by reducing the finger spacing of IDT electrodes. However, when a passband frequency greater than 1 GHz is to be achieved by such spacing control, high-priced equipments such as a stepper and scanner should be used, and also need to utilize a shorter wavelength UV source and a special photoresist.
To avoid these problems, Japanese Patent Laid-open Publication No. sho 64-62911 discloses the use of a piezoelectric film deposited on sapphire or diamond as a material for increasing the velocity of the SAW, and U.S. Pat. No. 5,497,726 discloses a method of manufacturing a SAW element using a diamond substrate and at least one copper electrode. However, the SAW elements employing these materials are not amenable to commercialization due to high costs of the substrate materials.
Therefore, there has existed a need to develop a novel and economic substrate suitable for a high frequency and low loss SAW filter having broad bandwidth.
It is, therefore, a primary object of the invention to provide a novel piezoelectric substrate material suitable for preparing a high frequency surface acoustic wave filter having a low insertion loss, broad bandwidth in high frequency region.
In accordance with an aspect of the present invention, there is provided a surface acoustic wave filter comprising a piezoelectric single crystal substrate and two interdigital transducer (IDT) electrodes formed thereon, characterized in that the substrate has the composition of formula (I):
x(A)y(B)z(C)xe2x80x94p(P)n(N)xe2x80x83xe2x80x83(I)
wherein,
(A) is Pb(Mg1/3Nb2/3)O3 or Pb(Zn1/3Nb2/3)O3,
(B) is PbTiO3,
(C) is LiTaO3,
(P) is a metal selected from the group consisting of Pt, Au, Ag, Pd and Rh,
(N) is an oxide of a metal selected from the group consisting of Ni, Co, Fe, Sr, Sc, Ru, Cu and Cd,
x is a number in the range of 0.65 to 0.98,
y is a number in the range of 0.01 to 0.34,
z is a number in the range of 0.01 to 0.1, and
p and n are each independently a number in the range of 0.01 to 5.